A decades-long study of introduced voles on the Norwegian islands of Svalbard is helping to answer a longstanding puzzle of Arctic ecology — what drives the well-established population cycles of small Arctic mammals, such as voles and lemmings. These plant-eating rodents are among the most populous Arctic mammals. The results suggest the importance of predators as a primary factor driving the cycles, and shows that bottom-up, herbivore-plant interactions fail to generate their usual population cycles.